The bromodomain and extraterminal (BET) family of proteins (BET proteins) are readers of the epigenetic code that couple acetylation of lysine residues on histones to changes in chromatin structure and gene expression. The BET family includes BRD2, BRD3, BRD4, and BRDT, all of which are widely expressed across diverse tissues, with the exception of BRDT, whose expression is restricted to the testes. See Wu, S. Y. & Chiang, C. M., J. Biol. Chem., 282: 13141-13145 (2007). Each BET family member contains tandem bromodomains in the N-terminal regions that specifically bind acetylated lysine residues in histones H3 and H4. Id. Once bound to histones, BET proteins recruit protein complexes that modulate gene transcription either directly, such as transcriptional activators or repressors, or indirectly such as chromatin remodeling complexes. BRD4 is the most well studied member of the BET family and is known to preferentially recognize tetra-acetylated histone H4 epigenetic marks. See Filippakopoulos, P., et al., Cell, 149: 214-231 (2012). BRD4 recruits the p-TEFb complex to nucleosomes, which in turn phosphorylates the C-terminal tail of RNA polymerase II and increases the transcriptional elongation of neighboring genes. See Yang, Z., et al., Mol. Cell Biol., 28: 967-976 (2008); Urano, E., et al., FEBS Lett., 582: 4053-4058 (2008).
The epigenetic code, including histone acetylation, is highly perturbed in many pathological disease states, resulting in the aberrant expression of genes that control cell fate, cell differentiation, cell survival, and inflammatory processes. See, e.g., Cohen, I., et al., Genes Cancer, 2: 631-647 (2011); Brooks, W. H., et al., J. Autoimmun., 34: J207-219 (2010); Wierda, R. J., et al., J. Cell Mol. Med., 14: 1225-1240 (2010); Shirodkar, A. V. & Marsden, P. A., Curr. Opin. Cardiol., 26: 209-215 (2011); Villeneuve, L. M., et al., Clin. Exp. Pharmacol. Physiol., 38: 401-409 (2011). BET proteins including BRD4 have been identified as important mediators of altered gene expression profiles found in numerous diseases including cancer, diabetes, obesity, atherosclerosis, cardiovascular and renal disorders, and viral infection. See Muller, S., et al., Expert Rev. Mol. Med., 13: e29 (2011); Zhou, M., et al., J. Virol., 83: 1036-1044 (2009); Chung, C. W., et al., J. Med. Chem., 54: 3827-3838 (2011). For example, MYC has been implicated in the majority of human cancers and BET proteins have been identified as regulatory factors of c-Myc; inhibition of BET proteins, including BRD4, has been shown to downregulate MYC transcription. See Delmore, J. E., et al. Cell, 146, 904-17 (2011); Lovén, J. et al., Cell, 153, 320-34 (2013). Inhibitors and modulators of BET proteins, including BRD4, are therefore needed.